This invention relates generally to electrical tools such as powered screw drivers, impact wrenches or the like, and more particularly to such tools having a clutch.
Conventional electrical tools used for driving screws, nuts or the like have a clutch interposed between a motor shaft and a driven shaft connected to a bit so that a desired tightening torque is obtained with the adjustment of the force of a coil spring or the like. More specifically, such a clutch is used for terminating the transmission of the rotational force from the motor through disengagement thereof when the tightening torque exceeds a predetermined value. Furthermore, a limit switch associated with the clutch is used for triggering a self-holding switching circuit so that both terminals of the motor are short-circuited for effecting dynamic braking. As a result, the motor is stopped. At the time of motor stopping, the clutch is usually put in engaged state again due to rotational inertia. However, the clutch sometimes remains in disengaged state.
When a power switch of such a conventional tool is turned on again under a condition where the clutch is disengaged, since the limit switch is in a closed state, the switching circuit operates so that dynamic braking is performed. For this reason, therefore, it has hitherto been necessary to manually rotate the driven shaft to put the clutch in an engaged state before the main switch is turned on. Such manual operation required sometimes prior to restarting the motor is troublesome, and therefore, easy-handling tools have been desired.